Conventionally, methods in which a sensor module having a configuration in FIG. 11A is used (for example, see PTL 1) and a method in which a sensor module having a configuration in FIG. 11B is used (for example, see PTL 2) are known as a method for detecting a heat flow flowing out from a body surface and measuring (calculating) a core body temperature from a detection result.
For use of one sensor module in FIG. 11A, namely, a heat flux sensor in which a temperature sensor is attached to each of top and bottom surfaces of a heat insulator, a core body temperature Tb is calculated using the following equation (1) from a temperature Ta measured with the temperature sensor on the top side of the heat insulator and a temperature Tt measured with the temperature sensor on the bottom side of the heat insulator.Tb=(Tt−Ta)Rx/R1+Tt  (1)
Where R1 and Rx are a heat resistance of a heat insulator and a heat resistance of a subcutaneous tissue, respectively.
In the internal temperature calculating method in which the sensor module in FIG. 11A is used, basically fixed values are used as the heat resistances R1 and Rx. However, because the heat resistance Rx depends on a location or an individual, a measurement error is included in the core body temperature Tb, which is calculated from the equation (1) using the fixed value as the heat resistance Rx, according to a difference between the heat resistance Rx used and the actual heat resistance Rx. Therefore, sometimes time changes of the temperatures Tt and Ta are measured, and the heat resistance Rx is calculated from the measurement result (see PTL 1).
In the case that an internal temperature is calculated with the sensor module in FIG. 11B, a temperature difference expressing a heat flux from the body surface is measured with each of two heat flux sensors having different heat resistances of the heat insulators. The following two equations can be obtained when the temperature difference is measured with the two heat flux sensors having different heat resistances of the heat insulators.Tb=(Tt−Ta)Rx/R1+Tt  (2)Tb=(Tt′−Ta′)Rx/R2+Tt′  (3)
Where Ta and Ta′ are temperatures measured with the temperature sensors on the top sides of the right and left heat flux sensors in FIG. 11B. Where Tt and Tt′ are temperatures measured with the temperature sensors on the bottom sides of the right and left heat flux sensors in FIG. 11B. R1 and R2 are heat resistances of heat insulators of the heat flux sensors as illustrated in FIG. 11B.
In the case that R1 and R2 are known numbers, only Rx and Tb are unknown numbers in the equation (2). Accordingly, the core body temperature Tb can be obtained from the equations (2) and (3).
In the case that the internal temperature is calculated using the sensor module in FIG. 11B, the core body temperature Tb is measured (calculated) by the principle.